Findings from Nansen survey on the Sierra Leone Rise will support decision-making for the long-term protection of vulnerable ecosystems in the Central Atlantic

22/02/2022

Over the past month, the research vessel Dr Fridtjof Nansen has been on a scientific expedition to study the presence of Vulnerable Marine Ecosystems (VMEs) on the Sierra Leone Rise in the eastern central Atlantic that may need to be protected from fishing activities. In total, five seamounts – Annan, Falsos, Perdido, La Meseta and Rompetodo – were explored with the use of specialized scientific equipment to document the physical and chemical conditions of the water masses surrounding the seamounts, and the benthic species and ecosystems that live on them. The survey also provided an opportunity to assess the degree of human impact on these remote seamounts, including the presence marine debris or litter, lost fishing gear and signs of physical impacts from fishing activities on the seabed.

Seamount ecosystems provide habitats for a variety of ocean wildlife, some of which are particularly vulnerable to external impacts as they are slow to recover, and preserving them is crucial to achieve sustainable use of resources and biodiversity conservation. Mapping vulnerable ecosystems is a first step towards ensuring their protection, by either closing them off or restricting them from harmful fishing practices. ”This expedition documented extensive coral gardens and sea-pen fields at the summit and shallow terraces leading up to the summits at La Meseta, Annan and Rompetodo, and the information collected will allow us to better understand these habitats and assess the external impacts”, said Dr Tina Kutti from the Norwegian Institute of Marine Research (IMR) and the cruise-leader of the expedition.

During the expedition, 20 scientists and technicians from west Africa (Cabo Verde, Senegal, Mauritania, and Guinea), as well as Spain and Portugal supported by researchers from IMR, explored the bottom habitats and studied the occurrence of deep-sea sponge aggregations, cold-water coral reefs and coral gardens found on the summits and flanks of the seamounts from 200 to 1500 meters depth.

All the seamounts that were surveyed had a very complex topography with steep walls and overhangs making them ideal areas for the colonization of coral and sponges, but also challenging to map. For example, the summit of the Rompetodo seamount is characterized by an asymmetrically collapsed top creating a very steep wall between 700 and 300 m depth on its south-western flank. “Rompetodo was named by Spanish fishermen, and the name means something like “destroys everything”, and looking at the digital elevation model of this seamount you could totally understand why” said Péricles Silva from the Ocean Science Centre Mindelo (OSCM) in Cabo Verde and the local co-cruise leader.

A digital elevation model of the Rompetodo seamount. © EAF-Nansen Programme

While exploring the wall of the Rompetodo seamount, mixed coral and sponge gardens with strikingly large Leiopathes black corals were discovered. The discoveries also unveiled coral colonies that exceeded two meters in width with the largest coral measuring almost three meters across. Black corals have an internal skeleton that develop rings as it grows, just like a tree, that can be counted. In other areas, these growth bands have been documented and it is estimated that Leiopathes black corals can live up to 2000-4000 years. The ages of the corals at Rompetodo were not assessed, but based on their sizes they are most likely very old.

Strikingly large Leiopathes black corals were discovered.
© EAF-Nansen Programme

Lost fishing gear and other human debris were observed during the underwater video surveys and the catches from two beam trawl hauls carried out on the flat-topped summit of the Rompetodo seamount were composed of over 50 percent litter. © EAF-Nansen Programme

In addition to the various formations of coral and sponges, there were other findings too: “to our surprise, mixed coral and sponge garden were observed throughout nearly all deeper water layers - from 400 meters and deeper. It is therefore likely that the spatial extent of these type of habitats on these seamounts is tremendous”, Dr Kutti explained. At a first glance, the five different seamounts seem to share the same biological and abiotic features, but it is possible that the post survey analysis of the data will reveal subtle differences in community composition, species distribution and ecosystem characteristics among the different seamounts. “We measured several different water mass properties that we know act as important drivers for coral and sponge distribution and health state, such as food particle availability and aragonite saturation state, said Pericles Silva.

Dr Eduardo Balguerias Guerra from the Instituto Español de Oceanografía (IEO) identifying fish caught on the Sierra Leone Rise survey. © EAF-Nansen Programme

All in all, Dr Kutti was pleased with the expedition findings and explained that “this survey presented a unique opportunity to expand our knowledge about the critical environmental factors regulating the distribution of species that form deep sea coral gardens, sponge aggregations and coral reefs often associated with seamounts.” “Next step will be to evaluate whether management actions are needed to safeguard these vulnerable species and habitats from future human impacts such as destructive fishing practices”, she continued.

This research expedition was a joint effort between the EAF-Nansen Programme and the Fishery Committee for the Eastern Central Atlantic (CECAF). Post survey analysis will enhance the understanding of biodiversity and Central Atlantic ecosystems and the data will support science-based decisions for the long-term protection of VMEs and sustainable use of resources in this part of the ocean.